Like any good physicst, having dabbled in the time-independent case, I then thought I'd tried to add time-dependence and see what happens. So after looking at the usage of knitting books, I turned to study knitting magazines.
This thread popped up on Ravelry at the beginning of November, after a user received a copy of the Holiday 2014 Vogue Knitting magazine and started a discussion about how useful people found the magazine. Several commenters said that they felt that knit magazines contained fewer useful patterns or had changed focus. So I thought I would adapt the code I developed to study book use to see if magazine use had decreased through time.
As always, adding time dependence makes everything messier.
The top left plots shows the total number of projects (in any state: cast on, in progress, completed, or frogged) from patterns in a given issue of Interweave Knits or Vogue Knitting. Magazines published between January 2005 and December 2012 were included. I made this top plot to get a sense of the over-all use of these magazines, and one can see that Interweave Knits has tended to produce more patterns that knitters turn into projects than Vogue Knitting has. The large spikes are from the extremely popular patterns; the biggest spike in that plot is from the Swallowtail Shawl, for example. The number of projects does decrease as we move forward in time, but that's to be expected as knitting is slow and there are plenty of knitters that would wait to cast on something until after finishing other projects.
The top right and bottom left plots show the average number of projects per user and the average number of patterns per user, respectively. Here the two lines are jumping around enough that its hard to draw any strong conclusions about trends, particularly from the average number of patterns. I can see that there isn't a clear difference between the use of either magazine, as both the blue and the red lines behave similarly. I also don't see any seasonal trend; Interweave Knits publishes four times a year and Vogue Knitting publishes five, so a season trend would be a dip or a spike every four or five points. There does seem to be a slight decline through in the average number of projects that I don't see in the average number of patterns, which implies that making repeated projects from the same pattern is fairly common.
The black line on the plots corresponds to May 2007, which is when Ravelry launched the beta version of its website. Knitting is spread out enough in time that I can't say that data would degrade in quality or accuracy before that point, since it would not be surprising for a knitter to make a project in 2010 from a pattern published in 2004. However, projects that were started before Ravelry launched and grew a large user base are probably less likely to be included, which would mean that early interest in patterns published before 2007 is less likely to be recorded on the website.
The bottom right plots shows the usage, or what fraction of each magazine's patterns are used. Here the two magazines clearly separate, as Interweave Knits has an average usage of about 5% while Vogue Knitting's sits around 3%. This is an effect of the relative size of the magazines, as Vogue Knitting tends to include a few more patterns per issue than Interweave Knits does.
In conclusion, I see no evidence that the overall use of these magazines is decreasing, though that is perhaps masked by the time of Ravelry's launch and the slow turn-around between a pattern's publishing and people actually making something from it. I see no evidence that those that use the magazines are decreasing the amount of material they use from each issue. It looks like readers are still happy with the material in Interweave Knits and Vogue Knitting, or that readers that leave are being replaced by those that join.
I should point that this could be a bit misleading as the average numbers of projects and patterns per user can't go below 0; someone who makes nothing from one of these magazines isn't included in these statistics. I could perhaps track specific users' projects from magazine to magazine, which would give more information on how individuals make use of these magazines.